In theory, threesomes are fantastic. Three's not a crowd, it's a hot, happening party. There's always a chance to be the centre of attention, to be serviced by two pairs of roaming hands and roving lips. But in practice, three-ways are rubbish.

There's a reason Sartre's version of hell in No Exit had three people stuck in a room together for eternity. It's hard enough to get one person to live up to the porn script you've written in your head, but two? Fuggedaboutit. There are more elbows to clank, heads to bump, and orifices to accidentally emit impolite noises.

But the problems don't end with the physical. The two people of the same sex invariably end up checking each other out - and not in a "Mmm, I like what you've got" way, but in a "Hmmm, what have you got that I haven't?" way. Insecurity begets competition, which begets bad vibes.

Even as piggy in the middle, you'll soon find guilt creeping into your loins. "Damn, this feels good, but I want to give." And if you're shy, the spotlight is never very flattering: "Damn, this feels good, but I wish these two would stop staring at me." And if you suffer from performance anxiety, you're sure to overanalyse: "Damn, this feels good, I should be moaning louder, climaxing sooner..."

If you're not the centre of attention, you're going to feel left out. If you thought being picked last for rounders in PE was painful, wait until you have to sit on the sidelines while the other two "have a moment".

With three people, someone is always an observer. During a two-person hook-up, it's easy to get lost in the moment, to project your fantasies on to the other. But threesomes can only ever be recreational, like an athletic event - but with no winners.